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Marshall fixed bias

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2/17/1998 2:02 AM

Marshall fixed bias

Hi, this is my first try with this mailing
thing, I hope I get through.
I have a Marshall Super Bass 100W plexi,
four EL-34 and a fixed bias (I think).
The problem is that it distorts all the time
I have measured the bias -90 volts, I guess
it is a bit to high, on the schematic I have
available the bias should be inbetween -60
and -80 volts. Today there is no fixed
resistor and pot in series to ground, I have
tried with a 47k and 22k pot and the bias
dropped to -44 but then the tubes on one
side almost cooked. The question is what
values for the fixed and pot and can it be
more to it than this (maybe a coupling cap)
Best regards

2/17/1998 3:17 AM

be carefull! i know you already know that, but...you want to stay around -37v to -43v with your bias number. yes, you can go much higher, but to get more a versitile sound, tighter highs and lows, stick to the sugguested settings. you'll want to raise your resistor before the bias pot to slightly higer values until you get in the ballpark you're looking for. one thing i've learned is that going from 6550's to el34's, you need to change that resistor from 150k(or120k) to 220k or so, half watt. maybe this will help...mike

2/17/1998 5:35 AM

Thanks for your quick reply, but I have tried
two things:

one, have the original(for me) wiring then I
get almost -100 volts bias, this way I get
an ugly distortion almost immediately.

two, install a 47k fixed resistor in series
with a 22k pot, then I get -44 volts max,
this way the tubes in one side get overheated.

So my question had two parts
one, the recommended voltage and current for
the EL-34
two, can it be that there is something else
too, a coupling cap maybe

Thanks again

2/17/1998 6:42 AM
J Fletcher	Could be a leaky cap on the side that gets too hot.Check the dc voltages at the power tube grids.Also check the bias feed resistors(220k from grid to bias supply)Proper bias voltage should be in the -36v to -42v range for EL34's,depending on the plate voltage,tubes,and desired tone.

2/17/1998 7:07 AM
Nils	Thanks again for great service, I just wonder if you can explain why they have stated -60 to -80 volts in the schematic, that I have. The plate voltage is 560 volts in the same schematic and I have measured it to 530 volts. Is this because they had different tubes 30 years ago or other criterias.

2/17/1998 8:21 AM

If the schematic says -60 to -80,then I'm wrong with my estimate.Didn't realize you had such high plate voltages,I was figuring somewhere around 450volts.Does your schematic show 6550 or EL34 tubes?The 6550's require more a more neg voltage than EL34's.That sounds too high of a bias voltage for EL34's to me,but I'm kind of in unfamiliar territory here.I would guess more in the -46 to -56 range,but that's a guess.If one pair of tubes runs way hotter than the other pair though,sounds like a problem with that pair.Did you check the bias voltage at the tube socket?

2/19/1998 10:51 AM
Derrick	Maybe your tube are not mached close enough per side.

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